In synchronous communications systems, it is typically necessary to derive a clocking signal from a reference signal. When the reference signal frequency is an integral multiple (N) of the desired signal frequency, the latter can be obtained as the output of a divide-by-N counter that is clocked by the reference signal. On the other hand, when the desired signal frequency is an integral multiple of the reference signal frequency, the former can be generated using a phase-locked loop. In many applications, there is oftentimes not a simple integral relationship between the reference signal frequency and the desired signal frequency. Instead, the reference signal frequency is some fractional multiple of the desired signal frequency, where the fractional multiple is a noninteger and is either less than or greater than 1. Moreover, in such applications, the prior art technique used to synthesize the desired signal frequency from the reference signal frequency is a function of the value of the fractional multiple.
Where the fractional multiple is less than 1, the desired signal frequency can be readily obtained from the output of a fractional multiplier which is clocked by the reference signal. Problems, however, arise when the fractional multiple is greater than 1 and, in general, are due to either limitations in the operating characteristics of available circuit devices and/or the cost of obtaining devices with the required operating characteristics. These problems are particularly burdensome in telecommunications applications where the required clocking signals for circuit devices are derived from reference signals available in the transmission line. These clocking rates have, in general, increased and are greater than and a fractional multiple of the available reference signal frequency. Synthesizing the desired signal frequency in such applications in a manner which meets the required system objectives, such as jitter and the like, has been extremely difficult and, at times, impossible to obtain using prior art techniques.
It would therefore be desirable if a signal synthesis technique could be developed which would overcome these limitations.